It has recently been discovered that certain pyranone compounds inhibit retroviral protease and thus they are useful for treating patients infected with human immunodeficiency virus (HIV) which results in acquired immunodeficiency syndrome (AIDS). In particular, the pyranone compound of formula I has been found to be especially effective as an inhibitor of retroviral protease. 
Several emulsion formulations of HIV drug are available in the prior art. However, these formulations require synthetic surfactants as emulsifiers to stabilize the compositions. For example, International publication WO 99/06043 discloses a self-emulsifying formulation which comprises the above pyranone compound of formula I, a mixture of diglyceride and monoglyceride, one or more solvents and one or more surfactants. Another example, WO 99/06044 discloses a discloses a self-emulsifying formulation which comprises the above pyranone compound of formula I, a basic amine, one or more solvents and one or more surfactants. Unfortunately, the catalog of suitable synthetic surfactants is limited. Although their potential scope is considerable, extensive toxicological studies are generally necessary to prove the harmlessness of the surfactants used in an emulsion formulation. Especially, the potentially adverse effect of surfactants on the newborn or small children is of major concern in today's society. Newborn and small children generally are more susceptible to chemical agents than adults.
Recognizing the potential problems, the present invention is directed toward pharmaceutical compositions in the form of emulsions which use natural or nontoxic lecithin as an emulsifier. The formulations of the present invention do not contain synthetic surfactants and, therefore, they are much safer for adult and pediatric use. It has also been discovered that lecithin dramatically increases the solubility of pyranone compound of formula I in oils. As a result, a high concentration of an emulsion formulation with high oral bioavailability can be prepared. Most surprisingly, the formulations of the present invention produce a prolonged blood level curve. Therefore, they have the potential for sustained release and high trough levels in the clinic.